1. Field of the Invention
The present invention relates to a permanent magnet motor, and more particularly to a structure of the permanent magnet motor with higher performance in which the cogging torque is reduced and the torque ripple is also reduced.
2. Description of the Related Art
In recent years, there is a demand for electric motors having small cogging torque and torque ripple for various purposes. Conventionally, one method of reducing the cogging torque of a permanent magnet motor has been proposed in which auxiliary grooves are formed on a face of each magnetic pole piece of the stator opposed to the permanent magnets of the rotor, though not shown, as disclosed in JP-A-10-42531, or a skew is provided by the amount of a period of cogging torque, as disclosed in JP-A-8-223832, for example.
In the conventional permanent magnet motor, as described above, the auxiliary grooves are formed on a face of each magnetic pole piece of the stator opposed to the permanent magnets of the rotor, and a skew is provided by the amount of a period of cogging torque, to reduce the cogging torque. However, when a skew angle provided at this time is inappropriate to reduce the torque ripple, the cogging torque and the torque ripple can not be reduced concurrently. For example, in a six-pole, nine-slot electric motor, if two auxiliary grooves are formed on a face of each magnetic pole piece of the stator, there is the almost equivalent effect where the number of slots in the stator is virtually 27. Then, the number of cogging torque ripples per rotation of the rotor is equal to 54, which is a least common multiple of the number of poles 6 and the virtual number of slots 27. And the period of cogging torque is converted to 20 degrees in terms of the electrical angle, and the cogging torque can be significantly reduced by the skew of an electrical angle of 20 degrees.
However, in order to reduce the torque ripple, the fifth and seventh harmonics of the induced voltage must be reduced.
Since the induced voltage usually contains more amount of the fifth harmonic than that of the seventh harmonic, it is effective for reducing the torque ripple to provide the skew at an angle where the fifth harmonic can be fully eliminated. However, the fifth harmonic can not be reduced even if the skew is provided at an electrical angle of 20 degrees, as described above. Therefore, in the case where this electric motor is driven by conducting a sinusoidal electric current, the torque ripple does not become small. There was the problem that even if the auxiliary grooves are formed or the skew angle is provided, it was difficult to reduce the cogging torque and the torque ripple.
This invention has been achieved to solve the above-mentioned problem, and it is an object of the invention to provide a permanent magnet motor of high performance in which the cogging torque is reduced and the torque ripple is also reduced.
According to a first aspect of the invention, there is provided a permanent magnet motor having a rotor with a plurality of permanent magnets disposed at a predetermined interval in a peripheral direction, and a stator with a plurality of magnetic pole pieces arranged at a predetermined spacing in the peripheral direction, the magnetic pole pieces of the stator being opposed to the permanent magnets of the rotor, wherein the auxiliary grooves are provided on a face of each magnetic pole piece of the stator that is opposed to the permanent magnets of the rotor, and a skew having an electric angle of 72xc2x0 is provided relatively between the rotor and the stator.
According to a second aspect of the invention, there is provided the permanent magnet motor, wherein a skew having an electric angle of 72xc2x0 is provided relatively between the rotor and the stator, where the number of poles is equal to 2N, and the number of slots is equal to 3N (N is an integer greater than or equal to 1).
According to a third aspect of the invention, there is provided the permanent magnet motor, wherein a skew having an electric angle of 72xc2x0 is provided relatively between the rotor and the stator, where the number of poles is equal to N, and the number of slots is equal to 3N (N is an integer greater than or equal to 2).
According to a fourth aspect of the invention, there is provided the permanent magnet motor according to claim 1, wherein a skew having an electric angle of 72xc2x0 is provided relatively between the rotor and the stator, where the number of poles is equal to 4N, and the number of slots is equal to 3N (N is an even number greater than or equal to 1).
According to a fifth aspect of the invention, there is provided the permanent magnet motor, wherein a skew is disposed on either one of the rotor and the stator.
According to a sixth aspect of the invention, there is provided the permanent magnet motor, wherein a skew is disposed on both the rotor and the stator.
According to a seventh aspect of the invention, there is provided the permanent magnet motor, wherein four auxiliary grooves are provided for every magnetic pole piece.
According to an eighth aspect of the invention, there is provided the permanent magnet motor, wherein the windings of the stator are concentrated.
According to a ninth aspect of the invention, there is provided the permanent magnet motor, wherein the permanent magnet of the rotor is magnetized in a radial direction.